(a) Field of the Invention
The field of the present invention is woven fabrics of synthetic materials for use under high temperature conditions as may be encountered in papermaking and other industrial processes.
(b) Description of the Prior Art
Woven fabrics fashioned into endless belts for conveying and guiding products under manufacture are used in various industrial processes. Both metal and synthetic materials have been used for these belts, but numerous processes involve high temperature and high moisture conditions which ordinary synthetic materials cannot withstand. In such cases metallic thread materials are commonly used, and typical of these are fine wires of brass, bronze or steel. The wires are woven to form a flat fabric and then seamed at the fabric ends to form endless belts. Steels can withstand temperatures up to about 1000.degree. F. (538.degree. C.), and the brasses and brass alloys can be used for temperatures up to about 600.degree. to 700.degree. F. (316.degree. to 371.degree. C.). Metal fabrics, however, are often difficult to handle, do not wear well, have poor flexure resistance and are prone to damage. They may also chemically interact with the product being conveyed, or can readily corrode under adverse environments. Thus metal fabrics have had severe limitations.
Two synthetic materials that have found some use in high temperature applications are a polymer of m-phenylenediamine and isophthaloyl chloride known as Nomex, and an aramid known as Kevlar, as reported in U.S. Pat. No. 4,159,618. These materials are twisted from multifilaments, or staple fibers into yarns, and are not available for applications where monofilament threads are preferred. Having a relatively rough, porous surface a multifilament can be difficult to keep clean in applications where contaminants are a problem, and for this reason Nomex and Kevlar yarns are sometimes coated with suitable resins to simulate monofilaments. These composite yarns can be woven or knitted into fabrics useful in such applications as conveying belts for dryer sections of a paper machine, where elevated temperatures are frequently encountered. However, under extended exposure to dry or moist heat there can be a severe loss in tensile strength, as further reported in said patent.
Another synthetic material that is woven from monofilaments into fabrics for use as industrial conveying and guiding belts is polyester. It has gained widely accepted usage in forming, press and dryer sections of papermaking machines because of its abrasion resistance, ability to flex, dimensional stability after being thermoset, chemical inertness, and ease of handling. Over the years techniques have been developed for weaving, thermosetting and seaming polyester threads and fabrics, so that this material can be readily handled in the manufacture of endless belts. Polyester consequently enjoys wide acceptance; however, this material has poor high temperature hydrolytic stability, and cannot be satisfactorily used under moist conditions at continuous, elevated temperatures. In papermaking applications, for example, it can be a limiting factor for the temperatures under which drying processes can be carried out, and where high temperatures are desired some other thread material must be resorted to.
In other manufacturing processes, too, such as in continuous drying or curing ovens for heat treating a product it is advantageous to employ conveying belts that can withstand high temperature and moisture conditions for the processing. In some installations coarsely woven metal belts, or belts constructed of metal links may be satisfactory, but where high speed operation or some other criteria dictates a different belt material, then there has not been a satisfactory answer for meeting belt requirements in high temperature applications. The present invention provides a response to these problems.